Here are five words you've heard before: "the world's fastest notebook GPU." Why, NVIDIA made just that claim two weeks ago, when it touted the GeForce GTX 580M as the nimblest card this side of Pluto. Not so fast, says AMD. The outfit just unveiled the Radeon HD 6990M with DirectX11 and HD3D support, and it insists this is the speediest GPU on the block -- specifically, up to 25 percent faster than any other GPU that's been announced to the public. And yes, AMD's well aware of that 580M. Just like NVIDIA came out swinging, making p
